Chapter 9: Virtual Memory
Chapter 9: Virtual Memory
 Background
 Demand Paging
 Copy-on-Write
 Page Replacement
 Allocation of Frames
 Thrashing
 Memory-Mapped Files
 Allocating Kernel Memory
 Other Considerations
 Operating-System Examples
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Objectives
 To describe the benefits of a virtual memory system
 To explain the concepts of demand paging, page-replacement
algorithms, and allocation of page frames
 To discuss the principle of the working-set model
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Background
 Virtual memory – separation of user logical memory from physical
memory.

Only part of the program needs to be in memory for execution

Logical address space can therefore be much larger than
physical address space

Allows address spaces to be shared by several processes

Allows for more efficient process creation
 Virtual memory can be implemented via:

Demand paging

Demand segmentation
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Virtual Memory That is Larger Than Physical Memory

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Virtual-address Space
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Shared Library Using Virtual Memory
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Demand Paging
 Bring a page into memory only when it is needed

Less I/O needed

Less memory needed

Faster response

More users
 Page is needed  reference to it

invalid reference  abort

not-in-memory  bring to memory
 Lazy swapper – never swaps a page into memory unless page will
be needed

Swapper that deals with pages is a pager
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Transfer of a Paged Memory to Contiguous Disk Space
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Valid-Invalid Bit

With each page table entry a valid–invalid bit is associated
(v  in-memory, i  not-in-memory)

Initially valid–invalid bit is set to i on all entries

Example of a page table snapshot:
Frame #
valid-invalid bit
v
v
v
v
i
….
i
i
page table

During address translation, if valid–invalid bit in page table entry
is I  page fault
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Page Table When Some Pages Are Not in Main Memory
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Page Fault
 If there is a reference to a page, first reference to that
page will trap to operating system:
page fault
1. Operating system looks at another table to decide:
 Invalid reference  abort
 Just not in memory
2. Get empty frame
3. Swap page into frame
4. Reset tables
5. Set validation bit = v
6. Restart the instruction that caused the page fault
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Page Fault (Cont.)

Restart instruction

block move

auto increment/decrement location
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Steps in Handling a Page Fault
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Performance of Demand Paging
 Page Fault Rate 0  p  1.0

if p = 0 no page faults

if p = 1, every reference is a fault
 Effective Access Time (EAT)
EAT = (1 – p) x memory access
+ p (page fault overhead
+ swap page out
+ swap page in
+ restart overhead
)
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Demand Paging Example
 Memory access time = 200 nanoseconds
 Average page-fault service time = 8 milliseconds
 EAT = (1 – p) x 200 + p (8 milliseconds)
= (1 – p x 200 + p x 8,000,000
= 200 + p x 7,999,800
 If one access out of 1,000 causes a page fault, then
EAT = 8.2 microseconds.
This is a slowdown by a factor of 40!!
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Process Creation

Virtual memory allows other benefits during process creation:
- Copy-on-Write
- Memory-Mapped Files (later)
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Copy-on-Write
 Copy-on-Write (COW) allows both parent and child processes to
initially share the same pages in memory
If either process modifies a shared page, only then is the page
copied
 COW allows more efficient process creation as only modified
pages are copied
 Free pages are allocated from a pool of zeroed-out pages
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Before Process 1 Modifies Page C
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After Process 1 Modifies Page C
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What happens if there is no free frame?
 Page replacement – find some page in memory, but not
really in use, swap it out

algorithm

performance – want an algorithm which will result in
minimum number of page faults
 Same page may be brought into memory several times
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Page Replacement
 Prevent over-allocation of memory by modifying page-fault service
routine to include page replacement
 Use modify (dirty) bit to reduce overhead of page transfers – only
modified pages are written to disk
 Page replacement completes separation between logical memory
and physical memory – large virtual memory can be provided on a
smaller physical memory
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Need For Page Replacement
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Basic Page Replacement
1.
Find the location of the desired page on disk
2.
Find a free frame:
- If there is a free frame, use it
- If there is no free frame, use a page replacement
algorithm to select a victim frame
3.
Bring the desired page into the (newly) free frame;
update the page and frame tables
4.
Restart the process
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Page Replacement
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Page Replacement Algorithms
 Want lowest page-fault rate
 Evaluate algorithm by running it on a particular
string of memory references (reference string) and
computing the number of page faults on that string
 In all our examples, the reference string is
1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
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Graph of Page Faults Versus The Number of Frames
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First-In-First-Out (FIFO) Algorithm

Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

3 frames (3 pages can be in memory at a time per process)


1
1
4
5
2
2
1
3
3
3
2
4
1
1
5
4
2
2
1
5
3
3
2
4
4
3
9 page faults
4 frames
10 page faults
Belady’s Anomaly: more frames  more page faults
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FIFO Page Replacement
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FIFO Illustrating Belady’s Anomaly
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Optimal Algorithm
 Replace page that will not be used for longest period of time
 4 frames example
1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1
4
2
6 page faults
3
4
5
 How do you know this?
 Used for measuring how well your algorithm performs
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Optimal Page Replacement
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Least Recently Used (LRU) Algorithm
 Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1
1
1
1
5
2
2
2
2
2
3
5
5
4
4
4
4
3
3
3
 Counter implementation

Every page entry has a counter; every time page is referenced
through this entry, copy the clock into the counter

When a page needs to be changed, look at the counters to
determine which are to change
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LRU Page Replacement
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LRU Algorithm (Cont.)
 Stack implementation – keep a stack of page numbers in a double
link form:


Page referenced:

move it to the top

requires 6 pointers to be changed
No search for replacement
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Use Of A Stack to Record The Most Recent Page References
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LRU Approximation Algorithms
 Reference bit

With each page associate a bit, initially = 0

When page is referenced bit set to 1

Replace the one which is 0 (if one exists)

We do not know the order, however
 Second chance

Need reference bit

Clock replacement

If page to be replaced (in clock order) has reference bit = 1
then:

set reference bit 0

leave page in memory

replace next page (in clock order), subject to same rules
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Second-Chance (clock) Page-Replacement Algorithm
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Counting Algorithms
 Keep a counter of the number of references that have been
made to each page
 LFU Algorithm: replaces page with smallest count
 MFU Algorithm: based on the argument that the page with
the smallest count was probably just brought in and has yet
to be used
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Allocation of Frames
 Each process needs minimum number of pages
 Example: IBM 370 – 6 pages to handle SS MOVE instruction:

instruction is 6 bytes, might span 2 pages

2 pages to handle from

2 pages to handle to
 Two major allocation schemes

fixed allocation

priority allocation
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Fixed Allocation
 Equal allocation – For example, if there are 100 frames and 5
processes, give each process 20 frames.
 Proportional allocation – Allocate according to the size of process
si  size of process pi
S   si
m  total number of frames
s
ai  allocation for pi  i  m
S
m  64
si  10
s2  127
10
 64  5
137
127
a2 
 64  59
137
a1 
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Priority Allocation
 Use a proportional allocation scheme using priorities rather
than size
 If process Pi generates a page fault,

select for replacement one of its frames

select for replacement a frame from a process with
lower priority number
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Global vs. Local Allocation
 Global replacement – process selects a replacement
frame from the set of all frames; one process can take a
frame from another
 Local replacement – each process selects from only its
own set of allocated frames
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Thrashing
 If a process does not have “enough” pages, the page-fault rate is
very high. This leads to:

low CPU utilization

operating system thinks that it needs to increase the degree of
multiprogramming

another process added to the system
 Thrashing  a process is busy swapping pages in and out
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Thrashing (Cont.)
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Demand Paging and Thrashing


Why does demand paging work?
Locality model

Process migrates from one locality to another

Localities may overlap
Why does thrashing occur?
 size of locality > total memory size
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Locality In A Memory-Reference Pattern
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Working-Set Model
   working-set window  a fixed number of page references
Example: 10,000 instruction
 WSSi (working set of Process Pi) =
total number of pages referenced in the most recent  (varies
in time)

if  too small will not encompass entire locality

if  too large will encompass several localities

if  =   will encompass entire program
 D =  WSSi  total demand frames
 if D > m  Thrashing
 Policy if D > m, then suspend one of the processes
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Working-set model
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Keeping Track of the Working Set
 Approximate with interval timer + a reference bit
 Example:  = 10,000

Timer interrupts after every 5000 time units

Keep in memory 2 bits for each page

Whenever a timer interrupts copy and sets the values of all
reference bits to 0

If one of the bits in memory = 1  page in working set
 Why is this not completely accurate?
 Improvement = 10 bits and interrupt every 1000 time units
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Page-Fault Frequency Scheme
 Establish “acceptable” page-fault rate

If actual rate too low, process loses frame

If actual rate too high, process gains frame
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Memory-Mapped Files
 Memory-mapped file I/O allows file I/O to be treated as routine
memory access by mapping a disk block to a page in memory
 A file is initially read using demand paging. A page-sized portion of
the file is read from the file system into a physical page.
Subsequent reads/writes to/from the file are treated as ordinary
memory accesses.
 Simplifies file access by treating file I/O through memory rather
than read() write() system calls
 Also allows several processes to map the same file allowing the
pages in memory to be shared
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Memory Mapped Files
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Memory-Mapped Shared Memory in Windows
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Allocating Kernel Memory
 Treated differently from user memory
 Often allocated from a free-memory pool

Kernel requests memory for structures of varying sizes

Some kernel memory needs to be contiguous
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Buddy System
 Allocates memory from fixed-size segment consisting of physically-
contiguous pages
 Memory allocated using power-of-2 allocator

Satisfies requests in units sized as power of 2

Request rounded up to next highest power of 2

When smaller allocation needed than is available, current
chunk split into two buddies of next-lower power of 2

Continue until appropriate sized chunk available
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Buddy System Allocator
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Slab Allocator
 Alternate strategy
 Slab is one or more physically contiguous pages
 Cache consists of one or more slabs
 Single cache for each unique kernel data structure

Each cache filled with objects – instantiations of the data
structure
 When cache created, filled with objects marked as free
 When structures stored, objects marked as used
 If slab is full of used objects, next object allocated from empty slab

If no empty slabs, new slab allocated
 Benefits include no fragmentation, fast memory request satisfaction
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Slab Allocation
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Other Issues -- Prepaging
 Prepaging

To reduce the large number of page faults that occurs at process
startup

Prepage all or some of the pages a process will need, before
they are referenced

But if prepaged pages are unused, I/O and memory was wasted

Assume s pages are prepaged and α of the pages is used


Is cost of s * α save pages faults > or < than the cost of
prepaging
s * (1- α) unnecessary pages?
α near zero  prepaging loses
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Other Issues – Page Size
 Page size selection must take into consideration:

fragmentation

table size

I/O overhead

locality
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Other Issues – TLB Reach
 TLB Reach - The amount of memory accessible from the TLB
 TLB Reach = (TLB Size) X (Page Size)
 Ideally, the working set of each process is stored in the TLB

Otherwise there is a high degree of page faults
 Increase the Page Size

This may lead to an increase in fragmentation as not all
applications require a large page size
 Provide Multiple Page Sizes

This allows applications that require larger page sizes the
opportunity to use them without an increase in
fragmentation
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Other Issues – Program Structure
 Program structure

Int[128,128] data;

Each row is stored in one page
 Program 1
for (j = 0; j <128; j++)
for (i = 0; i < 128; i++)
data[i,j] = 0;
128 x 128 = 16,384 page faults

Program 2
for (i = 0; i < 128; i++)
for (j = 0; j < 128; j++)
data[i,j] = 0;
128 page faults
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Other Issues – I/O interlock
 I/O Interlock – Pages must sometimes be locked into
memory
 Consider I/O - Pages that are used for copying a file
from a device must be locked from being selected for
eviction by a page replacement algorithm
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Reason Why Frames Used For I/O Must Be In Memory
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Operating System Examples
 Windows XP
 Solaris
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Windows XP
 Uses demand paging with clustering. Clustering brings in pages
surrounding the faulting page.
 Processes are assigned working set minimum and working set
maximum
 Working set minimum is the minimum number of pages the process
is guaranteed to have in memory
 A process may be assigned as many pages up to its working set
maximum
 When the amount of free memory in the system falls below a
threshold, automatic working set trimming is performed to
restore the amount of free memory
 Working set trimming removes pages from processes that have
pages in excess of their working set minimum
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Solaris
 Maintains a list of free pages to assign faulting processes
 Lotsfree – threshold parameter (amount of free memory) to begin
paging
 Desfree – threshold parameter to increasing paging
 Minfree – threshold parameter to being swapping
 Paging is performed by pageout process
 Pageout scans pages using modified clock algorithm
 Scanrate is the rate at which pages are scanned. This ranges from
slowscan to fastscan
 Pageout is called more frequently depending upon the amount of
free memory available
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Solaris 2 Page Scanner
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End of Chapter 9